Maize represents one of the main cultivar for food and energy and crop yields are influenced by soil physicochemical and climatic conditions. To study how maize plants influence soil microbes we have examined microbial communities that colonize maize plants grown in carbonate-rich soil (pH 8.5) using culture-independent, PCR-based methods. We observed a low proportion of unclassified bacteria in this soil whether it was planted or unplanted. Our results indicate that a higher complexity of the bacterial community is present in bulk soil with microbes from nine phyla, while in the rhizosphere microbes from only six phyla were found. The predominant microbes in bulk soil were bacteria of the phyla Acidobacteria, Bacteroidetes and Proteobacteria, while Gammaproteobacteria of the genera Pseudomonas and Lysobacter were the predominant in the rhizosphere. As Gammaproteobacteria respond chemotactically to exudates and are efficient in the utilization of plants exudate products, microbial communities associated to the rhizosphere seem to be plant-driven. It should be noted that Gammaproteobacteria made available inorganic nutrients to the plants favouring plant growth and then the benefit of the interaction is common.

SummaryIn silico models for Pseudomonas putida KT2440 metabolism predict 68 genes to be essential for growth on minimal medium. In this study a genomewide collection of single-gene P. putida KT2440 knockouts was generated by mini-Tn5 transposon mutagenesis and used to identify genes essential for growth in minimal medium with glucose. Our screening of the knockout library allowed us to rescue mutants for 48 different knockouts that were conditionally essential for growth on minimal medium. The in vivo screening showed that 24 of these mutants had a insertion in genes proposed to be conditionally essential based on in silico models, whereas another 24 newly implicated conditionally essential genes have been found. For 10 of the in silico proposed conditionally essential genes not found in the screening, knockout mutants were available at the Pseudomonas Reference Culture Collection. These mutants were tested for conditional growth on minimal medium, but none of them was shown to be essential, suggesting that the in silico proposal was inaccurate. Among the set of identified conditionally essential genes were a number of genes involved in the biosynthesis of certain amino acids and vitamins. Auxotrophs for all amino acids predicted by the in silico models were found and, in addition, we also found auxotrophs for proline, serine, threonine and methionine, as well as auxotrophs for biotin, nicotinate and vitamin B12 that were not predicted in silico. Metabolic tests were performed to validate the mutants' phenotypes. Auxotrophies for L-Arg, L-Leu, L-Pro and L-Cys were bypassed by external addition of the corresponding D-amino acids, suggesting the existence of number of D-to L-amino acid racemases encoded by the KT2440 genome. Therefore, the in vivo high-throughput analysis presented here provides relevant insights into the metabolic cross-road of biosynthetic pathways in this microorganism, as well as valuable information for the fine tuning of current in silico metabolic models.

This study presents the results of the physicochemical characterization of particulate matter associated with an important dust event from the Sahara area that occurred in the South of Spain in 2010. The chemical composition of the samples reflected the dominance of the crustal component of sand from the Sahara desert, although the presence of Mo, Ti, and V trace elements indicated that the dust contained industrial material; probably collected in its transport from Africa. Microbial biodiversity associated with the dust was low, but dominated by Firmicutes and Proteobacteria. Some Firmicutes (belonging to the genus Bacillus and Sporosarcina) were cultured on solid and liquid medium, which suggested that the transported microbes were alive or present as spores that germinated under favorable conditions. These cultivable microbes in the form of spores were highly resistant to desiccation, heat, and UV light.